Why am I getting artifacts in my baked normal map?
It seems as though, as of Blender 2.92, baking a tangent space normal map without using Selected to Active does not work correctly in the presence of modifiers. I have opened issue T87449 on the Blender bug tracker about this.
In the meantime, there are a few workarounds:

Apply modifiers before baking. This is probably the best workaround, since it’s easy to undo afterwards (or just temporarily duplicate the object for baking).

Bake an object space normal map instead of a tangent space one. This does not appear to be affected by the bug, though it incurs all the disadvantages of using an object space normal map.

Duplicate the object and use Selected to Active. This seems like strictly more work than just duplicating the object and applying all the modifiers, but it does seem to avoid the problem as well.


I decided to take the first approach, which, happily, produced a correct normal map.